Fructose and galactose metabolism

Question 1

What is the transporter for fructose into the enterocyte?

Answer 1

GLUT 5 with Na cotransport

Question 2

What is the transporter for fructose out of the enterocyte?

Answer 2

GLUT 2 and GLUT 5

Question 3

What is the typical pathway for digestion of fructose?

Answer 3

Fructose 1-phosphate pathway in liver

Question 4

What is an alternative pathway of glucose digestion?

Answer 4

By hexokinase to fructose-6P

Question 5

What is the first step of the fructose 1P pathway?

Answer 5

Fructose to fructose 1P by fructokinase using ATP

Question 6

What is the second step of the fructose 1P pathway?

Answer 6

Fructose 1P to glyceraldehyde and DHAP by fructose 1P aldolase
DHAP enters glycolysis or gluconeogenesis

Question 7

What happens to the glyceraldehyde produced in the fructose 1P pathway?

Answer 7

Glyceraldehyde to glyceraldehyde 3P by triose kinase using ATP.
This will enter glycolysis or gluconeogenesis

Question 8

What is the pathway to produce glucose in seminal plasma?

Answer 8

Sorbitol pathway

Question 9

What is the sorbitol pathway?

Answer 9

Glucose to sorbitol by aldose reductase using NADPH
Sorbitol to fructose by sorbitol dehydrogenase using NAD

Question 10

What tissues have low/absent sorbitol dehydrogenase?

Answer 10

Lens of eye, nerves, and kidneys

Question 11

What is the inheritance for hereditary fructose intolerance?

Answer 11

Autosomal recessive

Question 12

What enzyme is deficient in hereditary fructose intolerance?

Answer 12

Aldolase B

Question 13

What are the signs of hereditary fructose intolerance?

Answer 13

Recurrent vomiting, abd pain, and hypoglycemia after ingestion of fructose/sucrose

Question 14

What is the inheritance for essential fructosuria?

Answer 14

Autosomal recessive
Very rare

Question 15

What is the enzyme deficiency in essential fructosuria?

Answer 15

Hepatic fructokinase

Question 16

What are the symptoms of essential fructosuria?

Answer 16

Asymptomatic or mistaken for diabetes

Question 17

What is the transporter for galactose into the enterocyte?

Answer 17

SGLT-1 with Na contransport

Question 18

What is the transporter for galactose out of the enterocyte?

Answer 18

GLUT-2

Question 19

What is the first step of galactose metabolism?

Answer 19

Galactose to galactose-1P by galactokinase using ATP

Question 20

What is the second step of galactose metabolism?

Answer 20

Galactose-1P to Glucose-1P by GALT coupled with UDP-glucose to UDP galactose

Question 21

What are the two ways glucose 1P can be used?

Answer 21

To glucose 6P by phosphoglucomutase to enter glycolysis or gluconeogenesis
Into UDP-glucose by UDP glucose pyrophosphorylase using UTP and producing PPi

Question 22

What is the enzyme responsible for epimerization between UDP-glucose and UDP galactose?

Answer 22

UDP-galactose 4’ epimerase (GALE)

Question 23

How can UDP glucose be used?

Answer 23

Incorporated into glycogen
Synthesis of UDP galactose, making galactose nonessential
Uronic acid pathway

Question 24

When is lactose synthesized in the body?

Answer 24

During lactation

Question 25

How is lactose synthesized from UDP-galactose?

Answer 25

UDP-galactose to lactose by lactose synthase using glucose and producing UDP

Question 26

When is galactitol produced?

Answer 26

When there is an excess of galactose

Question 27

What happens to galactitol?

Answer 27

It is excreted in the urine

Question 28

What enzyme is deficient in classical galactosemia?

Answer 28

GALT

Question 29

What enzyme is deficient in nonclassical galactosemia?

Answer 29

Galactokinase

Question 30

What are signs of galactosemia?

Answer 30

Failure to thrive Vomiting and diarrhea following ingestion of milk Hepatomegaly and impaired liver function Hypergalactemia Excessive galactitol in urine Cataracts Hypoglycermia Mental retardation if untreated